This invention relates to an actuator for a differential locking mechanism where high pressure is delivered from a stationary source to a rotating source to actuate the differential lock.
Vehicle drive axles typically include a pair of axle shafts for driving vehicle wheels. The drive axle uses a differential to control input speed and torque to the axle shafts. Under ideal conditions, when the vehicle is driven along a straight path, the wheels will be turning at approximately the same speed and the torque will be equally split between both wheels. When the vehicle negotiates a turn, the outer wheel must travel over a greater distance than the inner wheel. The differential allows the inner wheel to turn at a slower speed than the outer wheel as the vehicle turns.
Power is transmitted from a vehicle drive-shaft to a pinion gear that is in constant mesh with a differential ring gear. The ring gear is bolted to a differential housing that turns with the ring gear. A differential spider having four (4) support shafts orientated in the shape of a cross, has four (4) differential pinion gears. One pinion gear is supported for rotation on each support shaft. Power is transferred from the differential housing to side gears that are splined to the axle shafts. The side gears are in constant mesh with the side differential pinion gears. The outer ends of the axle shafts are bolted to the brake drum hubs to which the wheels are also bolted.
When the vehicle is driven in a straight path the ring gear, differential housing, spider, and differential pinion gears all rotate as one unit to transfer power to the axle shafts. There is no relative movement between the differential pinion gears and the side gears. When the vehicle turns, the differential pinion gears rotate on their respective shafts to speed up the rotation of one axle shaft while slowing the rotation of the other axle shaft.
Often the differential includes a differential locking mechanism. When there are poor road conditions, i.e., slippery or rough surfaced roads, the locking mechanism allows maximum wheel and tire traction for improved control. If the differential does not have a locking mechanism and one tire is on ice, all of the torque and speed will be transferred to the wheel on ice. Thus, the tire just spins on the ice and the vehicle is prohibited from traveling forward. A locking mechanism allows the axle shafts to rotate at the same speed while transferring all available torque to the tire not on the ice. If the tractive effort at this tire is sufficient, the vehicle can be moved off of the ice. When the differential is locked, power is transmitted through the locked differential housing, gearing, and axle shafts rather than through the differential gearing only.
One type of differential locking mechanism is comprised of a wet disc clutch that locks the differential housing to the axle shafts. The wet disc clutch includes a plurality of stationary discs interspersed with rotating discs in a fluid chamber. A piston applies a force to the wet disc clutch to compress the rotating and stationary discs together to reduce rotational speed and allow the differential housing to be locked to the axle shafts.
One disadvantage with these wet disc clutches is system leakage. The leakage problem results from the pressure transfer from stationary members to rotating members to actuate the piston. Complicated leak-by recapture systems must be incorporated into the differential, which take up valuable packaging space and are expensive. The recapture system recovers the leaked fluid and returns it to a pump that is used for applying pressure to actuate the wet disc clutch. Another disadvantage is that high pressure is required at the input in order to have sufficient pressure at the actuating piston, which requires larger and more expensive pump systems.
Thus, it is desirable to have a simplified actuating mechanism for a differential lock that can deliver pressure from a stationary source to a rotating source while eliminating leakage. It is also desirable for the actuating mechanism to be able to have high pressure at the actuating piston with lower pressure at the input.
In a disclosed embodiment of this invention, an actuator delivers high pressure from a stationary source to a rotating source in order to actuate a differential locking mechanism. The actuator includes a fluid input, a stationary member with a fluid inlet for receiving fluid from the fluid input, and a rotating member. The rotating member has a fluid outlet and is supported on at least one bearing for rotation with respect to the stationary member. Fluid flows through the inlet to apply an actuating force to the rotating member via the stationary member. This causes the rotating member to displace a pre-determined amount of fluid from the fluid outlet to actuate the differential locking mechanism.
In a preferred embodiment, the differential locking mechanism is incorporated into a drive axle assembly. The drive axle includes an axle housing, a first rotatable axle shaft for driving a first wheel, and a second rotatable axle shaft for driving a second wheel. An axle differential controls rotational speed and torque supplied to the first and second axle shafts. The differential locking mechanism is selectively movable between an unlocked position and a locked position. In the unlocked position the first and second axle shafts can rotate at different speeds from each other, such as when the vehicle turns a corner. In the locked position the axle shafts are locked together for rotation at a common speed. A first volume of fluid applies an actuating force to the rotating member via the stationary member, which causes the rotating member to displace a second volume of fluid. This fluid displacement moves the locking mechanism from the unlocked position to the locked position.
In a further embodiment, the stationary member has a greater diameter than the rotating member does. This is advantageous because high pressure is provided at the locking mechanism with lower pressure at the input.
A preferred method for locking the differential includes steps of providing a stationary member having a fluid inlet, a rotating member having a fluid outlet, and a lock mechanism in fluid communication with the fluid outlet. Additional steps include applying a fluid force to the stationary member via the fluid inlet, actuating the rotating member with the stationary member, displacing fluid with the rotating member to a fluid passageway via the fluid outlet, and applying a fluid force to lock the lock mechanism via the fluid passageway.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.